I've used both (although, I've been in the katana more than in an F6, and I haven't been in the F6 for a long time, but...)
In my experience, the katana worked MUCH better for me and I simply didn't get back in the f6 for this exact reason.
I've got short arms and a big arch.. IIRC, you're a tall guy who's had issues benching in the past, yes?
I think if you can get into an F6 and learn it (just like any other shirt), you'd probably be able to make it work for you. Like you said, there's no reason to count it out.
IMO, the biggest difference between the shirts is where you need to touch. In the F6, the touch point will be higher than in the katana... and since you're in USAPL, this'll likely be better for you.
Also, the katana had more stopping power, IME.